Added replacement stub for cstdlib atexit() funciton.

This is an empty stub to simply allow use of complex types with a
non global static lifetime. For more complex handling the function
'atexit' can be redefined in user code.

For more information see:

https://github.com/arduino/Arduino/pull/2229
https://github.com/arduino/Arduino/issues/1919

cores/arduino/Arduino.h

@@ -115,6 +115,8 @@ typedef uint8_t byte;
 void init(void);
 void initVariant(void);
 
+int atexit(void (*func)()) __attribute__((weak));
+
 void pinMode(uint8_t, uint8_t);
 void digitalWrite(uint8_t, uint8_t);
 int digitalRead(uint8_t);

cores/arduino/main.cpp

@@ -19,6 +19,9 @@
 
 #include <Arduino.h>
 
+//Declared weak in Arduino.h to allow user redefinitions.
+int atexit(void (*func)()) { return 0; }
+
 // Weak empty variant initialization function.
 // May be redefined by variant files.
 void initVariant() __attribute__((weak));